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changed into a series of mingled fresh and brackish lagoons, which 

 finally, by continued terrestrial changes, were converted into a great 

 fresh-water lake, or, if we take the whole of Britain and areas now sea- 

 covered beyond, into a series of lakes. The occurrence of a few gen- 

 era or even species of fish and Crustacea common to the salt, brackish, 

 or fresh waters, does not prove that the passage-beds and those still 

 higher are truly marine. At the present day, animals commonly sup- 

 posed to be essentially marine are occasionally found inhabiting fresh 

 water. In the inland fresh lakes of Newfoundland, seals, which never 

 visit the sea, are common and breed freely. The same is the case in 

 Lake Baikal, 1,280 feet above the sea-level, in Central Asia; and, 

 though these facts bear but slightly on my present subject, seals being 

 air-breathing Mammalia, yet in the broad mouth of the Amazon, far 

 above the tidal influx of sea-water, marine mollusca and other kinds 

 of life are found, and in some of the lakes in Sweden there are marine 

 Crustacea. This may be easily accounted for in the same way that I 

 now attempt to account for analogous peculiarities in the Old Red 

 Sandstone. These Swedish lakes were submerged during the Glacial 

 period, and remained as deep basins while the land was emerging, and, 

 after its final emergence, the salt-waters of the lakes freshened so 

 slowly that some of the creatures inhabiting them had time by de- 

 grees to adjust themselves to new and abnormal conditions. 



In further illustration of the subject let us suppose a set of circum- 

 stances such as the following : By long-continued upheaval of the 

 mouth of the Baltic (a process now going on), its waters, already 

 brackish in the Gulfs of Bothnia and Finland, would eventually be- 

 come fresh, and true lacustrine strata over that area would succeed 

 and blend into the marine and brackish water-beds of earlier date. 

 Something of this kind I conceive to have marked the transition from 

 the Upper Silurian beds into the Old Red Sandstone. Again : if by 

 changes in the physical geography of the area, of a continental kind, 

 a portion of the Silurian sea got isolated from the main ocean, more or 

 less like the Caspian and the Black Sea, then the ordinary marine con- 

 ditions of the " passage-beds," accompanied by some of the life of the 

 period, might be maintained for what, in common language, seems to 

 us a long time. There is geological proof that the Black Sea was once 

 united to the Caspian, the two forming one great brackish lake. Since 

 they were disunited and the Bosporus opened, the Black Sea has, it 

 may be inferred, been steadily freshening ; and it is easy to conceive 

 that, by the reclosing of the Bosporus (a comparatively small geo- 

 graphical change), it might in the course of time again be converted 

 into a fresh lake. At present a great body of salt-water is constantly 

 being poured out through the Bosporus, and its place taken by the 

 fresh waters of the Danube and other rivers, while, owing to the uncon- 

 genial quality of the freshening sea, some of the Black-Sea shells are 

 strangely distorted, as was shown by Edward Forbes. 



